Cold storage material, refrigerator, device incorporating superconducting coil, and method of manufacturing cold storage material

ABSTRACT

A cold storage material, which has a large specific heat and a small magnetization in an extremely low temperature region and has satisfactory manufacturability, is provided, and a method for manufacturing the same is provided. Further, a refrigerator having high efficiency and excellent cooling performance is provided by filling this refrigerator with the above-described cold storage material. Moreover, a device incorporating a superconducting coil capable of reducing influence of magnetic noise derived from a cold storage material is provided. The cold storage material of embodiments is a granular body composed of an intermetallic compound in which the ThCr2Si2-type structure 11 occupies 80% by volume or more, and has a crystallite size of 70 nm or less.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of No. PCT/JP2019/037995,filed on Sep. 26, 2019, and the PCT application is based upon and claimsthe benefit of priority from Japanese Patent Application No.2018-185628, filed on Sep. 28, 2018, the entire contents of which areincorporated herein by reference.

TECHNICAL FIELD

Embodiments relate to a cold storage material to be used at an extremelylow temperature and a technique to which this cold storage material isapplied.

BACKGROUND

A superconducting electromagnet is used in a magnetic resonance imaging(MRI) system, a heavy particle beam accelerator which are operated in anextremely low temperature environment of several tens of K or less.Usually, this extremely low temperature environment is realized by acold storage type refrigerator represented by a Gifford-McMahon (GM)refrigerator.

Several types of cold storage materials, which have a large specificheat for each operating temperature region, are used in therefrigerators. In the GM refrigerators that are currently widely used,Cu mesh is used as the cold storage material for the first cold storagedevice, and spherical particles of Pb and Bi alloy are used as the coldstorage material on the high-temperature side of the second cold storagedevice, and particles of rare earth compounds such as Gd₂O₂S (GOS),HoCu₂, and Er₃Ni are used as the cold storage material on thelow-temperature side of the second cold storage device. Among such coldstorage materials, GOS has a high specific heat characteristic in atemperature region near 5K.

In order to synthesize an oxide cold storage material such as GOS, amulti-step process such as synthesis of raw materials, granulation,sintering at a high temperature, and spherical finishing by polishing isrequired.

Many refrigerators configured to achieve extremely low temperatures areused to cool superconducting coils. Thus, when the magnetization of thecold storage material is large, the cold storage material receives alarge force due to the magnetic field generated by the superconductingcoil and problems such as damage to the shaft containing the coldstorage material may occur, which reduces the reliability of therefrigerator. Although superconducting coils are used for MRI and thelike as described above, when the magnetization of the cold storagematerial is large, noise may be included in images due to magnetic noisederived from the cold storage material. Hence, the magnetization of thecold storage material is required to be small.

In refrigerators such as a GM refrigerator, a pulse tube refrigerator,and a Stirling refrigerator, high-pressure working gas fluidlyreciprocates through the gap between the cold storage materials packedin the cold storage device. Further, in a GM refrigerator and a Stirlingrefrigerator, the cold storage device filled with cold storage materialsvibrates. Thus, the cold storage materials are required to havemechanical strength.

In terms of manufacturing cold storage materials, it is preferred to useintermetallic compounds that can be manufactured by a simple process ofmelt-solidification. An oxide cold storage material such as GOS requirea multi-step manufacturing process such as synthesis of raw materials,granulation, sintering at a high temperature, and spherical finishing bypolishing. RCu₂X₂ (R═Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, X═Si, Ge) isknown to have a large specific heat at extremely low temperatures as achoice for the cold storage material of intermetallic compounds.

However, the RCu₂X₂ compound is produced by, for example, melting theraw materials under an arc melting method and then subjecting theobtained ingot to a uniform heat treatment at a high temperature for along time (for example, at 800° C. for one week). When ahigh-temperature and long-time heat treatment process is required aftermelt-solidification as described above, its cost increases in the caseof being applied to industrial mass production.

PRIOR ART DOCUMENT Patent Document

-   [Patent Document 1] JP H09-014774 A-   [Patent Document 2] JP H06-101915 A

Non-Patent Document

-   [Non-Patent Document 1] L. Gonedek, et. al., Acta Phys Pol A 122,    391 (2012).-   [Non-Patent Document 1] Y. Takeda, et. al., Phys. Soc. Jpn. 77,    104710 (2008).

SUMMARY Problems to be Solved by Invention

A cold storage material, which has a large specific heat and a smallmagnetization in an extremely low temperature region and hassatisfactory manufacturability, is provided, and a method formanufacturing the same is provided. Further, a refrigerator having highefficiency and excellent cooling performance is provided by filling thisrefrigerator with the above-described cold storage material. Moreover, adevice incorporating a superconducting coil capable of reducinginfluence of magnetic noise derived from a cold storage material isprovided.

Solution to Problem

The cold storage material of embodiments is a granular body composed ofan intermetallic compound in which a ThCr₂Si₂-type structure compoundoccupies 80% by volume or more, and has a crystallite size of 70 nm orless.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a model diagram of a ThCr₂Si₂-type structure showing a crystalstructure of a cold storage material according to the first embodiment.

FIG. 2 is a schematic diagram showing a grain shape of the cold storagematerial according to the first embodiment.

FIG. 3 is a cross-sectional view of a two-stage expansion type GMrefrigerator exemplified as the refrigerator according to the secondembodiment.

FIG. 4 is a cross-sectional view of an MRI apparatus exemplified as adevice incorporating a superconducting coil according to the thirdembodiment.

FIG. 5 is a graph showing measurement results by a powder X-raydiffraction method for Example 1 in the upper part and for ComparativeExample 1 in the lower part.

FIG. 6 is a graph showing the specific heat characteristics of Example 1and Comparative Example in an extremely low temperature region.

FIG. 7 is a graph showing measurement results by the powder X-raydiffraction method for Example 1 in the upper part and for ComparativeExample 2 in the lower part.

FIG. 8 is a table showing crystallite size, volume % of theThCr₂Si₂-type structure, proportion of each pulverized sample, peaktemperature of specific heat, and peak values of specific heat forintermetallic compounds of DyCu₂Ge₂, DyCu₂Si₂, GdCu₂Si₂, PrCu₂Si₂, andTbCu₂Si₂ in Example 1 to Example 7 and Comparative Example 1 toComparative Example 14.

FIG. 9 is a graph showing magnetization characteristics in an extremelylow temperature region of Example 1.

DETAILED DESCRIPTION First Embodiment

Hereinafter, embodiments will be described in detail. FIG. 1 is a modeldiagram of a ThCr₂Si₂-type structure 11 showing the crystal structure ofthe cold storage material according to the first embodiment. The coldstorage material according to the first embodiment is a granular bodycomposed of an intermetallic compound in which ThCr₂Si₂-type structure11 occupies 80% by volume or more, and has a crystallite size of 70 nmor less.

In the above-described ThCr₂Si₂-type structure 11, Th site 12 is atleast one element selected from the group consisting of La, Ce, Pr, Nd,Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y. In theabove-described ThCr₂Si₂-type structure 11, Si site 14 is at least oneelement selected from Si and Ge, and Cr site 13 is at least one elementselected from the group consisting of Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Ru,Rh, Pd, Ir, and Pt.

In a refrigerator such as a GM refrigerator described below, a workinggas such as He gas fluidly reciprocates through the gap of the coldstorage materials which fill the cold storage device, the lowtemperature generated by the compression/expansion cycle of the gas isstored in the cold storage material, and thereby, the refrigerator coolsdown from a room temperature to an extremely low temperature. Thus, thecold storage material to be mounted on the refrigerator is required tohave a large specific heat characteristic in the operating temperatureregion.

When the ThCr₂Si₂-type structure 11 occupies 80% by volume or more inthis intermetallic compound, the intermetallic a cold storage materialhaving a high specific heat characteristic in an extremely lowtemperature region can be obtained. If the proportion of theThCr₂Si₂-type structure 11 in the intermetallic compound is less than80% by volume, the specific heat characteristic is deteriorated in somecases compared with general substances listed as cold storage materialsin the extremely low temperature region. The volume % of theThCr₂Si₂-type structure can be calculated from the Rietveld analysis ofthe powder X-ray diffraction method and/or evaluation of the ratio ofthe phases of a plurality of fields of view by observation with ascanning electron microscope.

In a GM refrigerator and a Stirling refrigerator, the cold storagedevice filled with the cold storage material vibrates, and thus, thecold storage material is required to have mechanical strength. Thus,crystallite size of the cold storage material is as fine as 70 nm orless, which ensures excellent mechanical strength of the cold storagematerial. The crystallite size L is calculated by evaluating the peakwidth (half width) β in the X-ray diffraction pattern and usingScherrer's equation (Equation (1)). When the crystallite size is small,the half width of the X-ray diffraction pattern becomes large.

L=Kλ/(β cos θ)  (1)

(wherein K is the Scherrer constant and λ is wavelength of X-rays to beused)

The mechanical strength can be evaluated by a vibration test.

If the crystallite size of the cold storage material is larger than 70nm, the mechanical strength is reduced, the granules arefatigue-fractured and pulverized as the period of use elapses, and thepredetermined performance of the refrigerator cannot be maintained. Thecrystallite size is preferably 1 nm or more, more preferably 10 nm ormore.

FIG. 2 is a schematic diagram showing a grain shape of the cold storagematerial according to the first embodiment. As the particle size of thegranular body of the cold storage material, φmax is defined as thelength of the granular body in the longest direction and φmin is definedas the length of the longest portion in the direction perpendicular tothe longest direction. φmax and φmin are included in the range of 0.01mm to 1 mm, and more preferably, φmax and φmin are included in the rangeof 0.05 mm to 0.5 mm. When an area of a projected image 15 of this coldstorage material is defined as A and an area of the smallestcircumscribed circle 16 circumscribing this projected image 15 isdefined as M, the shape coefficient represented by M/A is included inthe range of 1.0 to 5.0 in every projection direction.

Since the particle size of the cold storage material is included in therange of 0.01 mm to 1 mm, in the refrigerator described below, the flowof the working gas (He gas) that fluidly reciprocates in the coldstorage device filled with the cold storage material is not obstructed,and satisfactory heat exchange between the working gas and the coldstorage material is achieved. When the particle size of the cold storagematerial is less than 0.01 mm (10 μm), the gap between the particles ofthe cold storage material (i.e., space through which the working gasflows) may be narrowed and the pressure loss of the gas may increase.When the particle size of the cold storage material is larger than 1 mm,the filling rate of the cold storage material in the cold storage devicemay decrease and the heat exchange between the working gas and the coldstorage material may decrease.

The production of such a cold storage material is performed by at leastgoing through a process of blending and melting the component elementsof an intermetallic compound, which can form the above-describedThCr₂Si₂-type structure 11, at its stoichiometric ratio and a process ofinjecting this molten liquid into a dynamic cooling medium and rapidlycooling and solidifying it into granules.

That is, the elemental metal blended to have the stoichiometric ratio ofthe ThCr₂Si₂-type structure 11 is melted by high frequency inductionheating or the like. Further, the molten metal is supplied to thesurface of a high-speed rotating body installed in the atmosphere ofvacuum or inert gas. This molten metal is finely dispersed by the motionof the rotating body and is simultaneously subjected torapid-solidification so as to form spherical granules. Additionally oralternatively, the above-described molten metal is made to flow out intoa vacuum or an atmosphere of inert gas, and a non-oxidizing atomizinggas is allowed to act on it. As a result, the molten metal is atomizedand dispersed, and at the same time, is rapidly cooled and solidified toform spherical granules.

Specific methods for rapidly cooling and solidifying the above-describedmolten metal include a rotary disc process (RDP) method, a single rollmethod, a double roll method, an inert gas atomizing method, and arotary nozzle method. According to these methods, the molten metal canbe rapidly cooled at a cooling rate of 10⁵ to 10⁶° C./s, and thus, anintermetallic compound having the ThCr₂Si₂-type structure can be readilyproduced in the form of granules at low cost. Details of therapid-solidification method for the molten metal are described inJapanese Patent No. 2609747.

When an intermetallic compound having a different magnetic-phasetransition-temperature is added to the intermetallic compound having theThCr₂Si₂-type structure, the specific heat characteristic per unitvolume of the cold storage material can be enhanced. For example, when aphase having an AlB₂-type structure and a LiGaGe-type structure ispresent in the intermetallic compound having the ThCr₂Si₂-typestructure, the specific heat in the 4K to 20K region can be increased.When a phase having a Gd₃Cu₄Ge₄-type structure is present in theintermetallic compound having the ThCr₂Si₂-type structure, the specificheat in the vicinity of 7K to 50K can be increased. However, when theamount of the phases excluding the ThCr₂Si₂-type structure is 20% byvolume or more, the volume specific heat derived from the ThCr₂Si₂-typestructure becomes small. When the intermetallic compound is composed ofphases having different crystal structures, the mechanical strength ofthe cold storage material can be increased.

Second Embodiment

FIG. 3 is a cross-sectional view of a two-stage expansion type GMrefrigerator exemplified as the refrigerator 30 according to the secondembodiment. The refrigerator 30 includes: a large-diameter firstcylinder 31; and a small-diameter second cylinder 32 that is coaxiallyconnected to the first cylinder 31. A first cold storage device 34 isdisposed in the first cylinder 31 so as to be able to reciprocate, and asecond cold storage device 35 is disposed in the second cylinder 32 soas to be able to reciprocate. A seal ring 36 is disposed between thefirst cylinder 31 and the first cold storage device 34, and a seal ring37 is disposed between the second cylinder 32 and the second coldstorage device 35.

A first expansion chamber 41 is provided between the inner wall of thefirst cylinder 31 and the connection portion of the first and secondcold storage devices 34 and 35. A second expansion chamber 42 isprovided between the second cold storage device 35 and the tip wall ofthe second cylinder 32. A first cooling stage 43 is formed at the bottomof the first expansion chamber 41, and a second cooling stage 44 beinglower in temperature than the first cooling stage 43 is formed at thebottom of the second expansion chamber 42.

In the first cold storage device 34, a first cold storage material 38such as a copper alloy mesh is accommodated under the state where apassage 33 of the working gas (He gas, and the like) is secured. As thefirst cold storage material 38, a stainless steel mesh may be usedinstead of the copper alloy mesh, and both of them may be used. Thesecond cold storage device 35 is filled with second cold storagematerials 40 in a form in which the passage 39 of the working gas issecured. Although a description has been given of the cold storagedevices 34 and 35 in which the first cold storage material 38 and thesecond cold storage materials 40 are packed separately, these may bepacked in one cold storage device.

The second cold storage materials 40 to be housed inside the second coldstorage device 35 are filled with a plurality of types of second coldstorage materials 40 a and 40 b such that these second cold storagematerials 40 a and 40 b are partitioned by a mesh 48. The filling rateof the second cold storage materials 40 a and 40 h in the spacepartitioned by the mesh 48 is preferably 50% to 75% in consideration ofthe fluidity of the working gas, and is more preferably 55% to 65%.

In the two-stage refrigerator 30, the working gas (He gas or the like)is compressed by a compressor 45 and supplied to the refrigerator 30through a high-pressure line 46. The supplied working gas passes throughthe gap of the first cold storage material 38 housed in the first coldstorage device 34 so as to reach the first expansion chamber 41, andthen cools the first cooling stage 43 by expansion. Next, the workinggas passes through the gap of the second cold storage materials 40housed in the second cold storage device 35 so as to reach the secondexpansion chamber 42, and then cools the second cooling stage 44 byexpansion.

The working gas having been made into low-pressure state passes throughthe second cold storage device 35 and the first cold storage device 34in this order (i.e., in the order opposite to the case of thehigh-pressure), and then is returned to the compressor 45 through alow-pressure line 47. Afterward, it is compressed by the compressor 45and the above-described cycle is repeated. The expansion of each of theexpansion chambers 41 and 42 is realized by the reciprocating operationof the cold storage devices 34 and 35. At this time, each of the coldstorage materials 38 and 40 exchanges heat energy with the working gasso as to accumulate and retain cold heat, and also performs heatregeneration.

Next, the cycle will be described focusing on the heat flow. Thehigh-pressure working gas to be supplied from the compressor 45 to therefrigerator 30 is at room temperature (about 300K). This working gas isprecooled by the first cold storage material 38 while passing throughthe first cold storage device 34, and then reaches the first expansionchamber 41. Afterward, the working gas expands in the first expansionchamber 41 so as to be further lowered in temperature and thereby coolsthe first cooling stage 43. Subsequently, the working gas is precooledby the second cold storage materials 40 while passing through the secondcold storage device 35, and then reaches the second expansion chamber42. Afterward, the working gas expands in the second expansion chamber42 so as to be further lowered in temperature and thereby cools thesecond cooling stage 44.

The working gas having been made into the low-pressure state passesthrough the inside of the second cold storage device 35 while storingcold heat in the second cold storage materials 40 (i.e., while theworking gas itself is being warmed).

Subsequently, the working gas passes through the inside of the firstcold storage device 34 so as to be warmed to near room temperature whilestoring cold heat in the first cold storage material 38 (i.e., while theworking gas itself is being warmed), and then passes through thelow-pressure line 47 so as to return to the compressor 45.

During the steady operation of the refrigeration cycle, a temperaturegradient occurs in the cold storage materials 38 and 40 inside the coldstorage devices 34 and 35.

In such a refrigeration cycle, the larger the specific heat of the coldstorage material at the operating temperature is, the higher the thermalefficiency of the working gas cycle becomes, which achieves lowertemperature and higher refrigeration performance.

In general, specific heat of a solid has the property of changingdepending on the temperature. Thus, in particular, in order to enhancethe heat recovery effect of the second cold storage materials 40, it iseffective to selectively dispose the second cold storage materials 40having satisfactory heat recovery characteristics in the respectivetemperature regions depending on the temperature gradient. Hence, thesecond cold storage device 35 is filled with a plurality of second coldstorage materials 40 (40 a, 40 b) having different heat recoverycharacteristics.

In order to obtain a satisfactory heat recovery effect, the followingcharacteristic are important. That is, the heat capacity (specific heat)of the cold storage material at the operating temperature of eachportion in the cycle process is large, and the heat exchange between thecold storage materials 40 and 38 and the working gas is satisfactory. Inthe first cold storage device 34, the temperature region from roomtemperature to 100K or less is the main operating temperature region,and thus, Cu having a large specific heat per unit volume in thistemperature region is selected. Further, Cu mesh is widely used as thefirst cold storage material 38 because mesh subjected to wire-drawingprocess is industrially easy to use.

Pb and Bi, which have a higher specific heat at 60K or less than Cu, areselected as the second cold storage material 40 a on thehigh-temperature side of the second cold storage device 35. The coldstorage material having the ThCr₂Si₂-type structure according to thefirst embodiment, which has a higher specific heat at 8K or less than Pband Bi, is selected as the second cold storage material 40 b on thelow-temperature side of the second cold storage device 35. Inconsideration of the temperature gradient inside the cold storagedevices 34 and 35, for the cold storage materials 38 and 40 of the GMrefrigerator, it is preferred to select and dispose substances having alarge volume specific heat in the operating temperature region of eachportion in the above-described manner. Note that the second cold storagematerial 40 a to be disposed on the high-temperature side of the secondcold storage device 35 is not limited to Pb and Bi. HoCu₂, Er₃Ni, andthe like may be disposed as the second cold storage material 40 a.Additionally, the second cold storage materials 40 are not limited tothe above-descried two layers but may be formed of three or more layers.

Further, the refrigerator equipped with the cold storage materialaccording to the first embodiment is not limited to the above-describedGM refrigerator. In refrigerators configured to generate an extremelylow temperature from a room temperature, such as a pulse tuberefrigerator, a Claude refrigerator, and a Stirling refrigerator, thecold storage materials are disposed at portions where a large thermalimpedance is required, such as the boundary region between the cold andhot portions to be caused by the compression/expansion cycle of theworking gas.

Third Embodiment

FIG. 4 is a cross-sectional view of a magnetic resonance imaging (MRI)apparatus 50 exemplified as a device incorporating a superconductingcoil according to the third embodiment. In the diagnosis by this MRIapparatus 50, a movable base (not shown) on which a subject 52 lies ismoved into a tunnel-shaped bore space 51. Further, a static magneticfield is applied by a first electromagnet 53, and a gradient magneticfield is applied by the second electromagnet 54.

Further, a radio wave is transmitted from an RF coil 55, and a magneticresonance signal is received as a response signal from the subject 52.Due to the gradient magnetic field, positional information on theposition where the response signal is generated is also received at thesame time. The received response signals are analyzed by a signalprocessing system (not shown) to reconstruct an internal image of thebody of the subject 52.

In the MRI apparatus 50 currently used as the mainstream, asuperconducting coil configured to generates a strong magnetic fieldsuch as 1.5 T and 3 T is used for the first electromagnet 53. Thestronger the magnetic field is, the better the S/N (signal/noise) ratioof the magnetic resonance response signal becomes, which enablesgeneration of a clearer image. As the superconducting coil to be usedfor the first electromagnet 53, a solenoid coil wound with metal-typelow-temperature superconducting wires such as NbTi and Nb₃Sn is usuallyused.

Since these wires need to be kept at the critical temperature of thesuperconducting transition or lower, the first electromagnet 53 isinstalled in a He bath 56 filled with liquid He that liquefies at 4.2K(about −269° C.) or less under 1 atm. Since the liquid He is rare andexpensive, an adiabatic vacuum layer 57 is provided on the outside ofthe He bath 56 in order to suppress evaporation of the liquid He.Further, in order to reduce the influence of heat intrusion from theenvironment (room temperature: about 300K) in which the MRI apparatus 50is installed, two radiation shields 58 and 59 are provided in theadiabatic vacuum layer 57. The shield 58 is cooled to about 4K and theshield 59 is cooled to about 40K by the installed refrigerator 30.

The refrigerator 30 is not limited to a particular one, and a GMrefrigerator and a JT refrigerator may be used in combination as therefrigerator 30. Additionally or alternatively, a refrigerator such as aGM refrigerator, a pulse tube refrigerator, a Claude refrigerator, and aSterling refrigerator is used alone as the refrigerator 30 in somecases. In particular, the GM refrigerator has significantly improved inrefrigeration performance by being equipped with a magnetic cold storagematerial in the 1990s, which has enabled generation of an extremely lowtemperatures below the liquid He temperature by using only the GMrefrigerator. Thus, the GM refrigerator is often used in the MRIapparatus 50, which is widely used at the time of filing of the presentapplication.

As shown in FIG. 4 , the first cooling stage 43 (FIG. 3 ) of the GMrefrigerator 30 is connected to the shield 59, and the second coolingstage 44 (FIG. 3 ) is connected to the shield 58. At the time of filingof the present application, GM refrigerators capable of stably obtaininga refrigeration capacity of 1 W or more at 4K are widespread. Thus, theheat invasion into the He bath 56 and the cooling by the GM refrigerator30 are balanced, and thereby, an extremely low temperature can bemaintained and evaporation of the liquid He can be almost completelysuppressed.

Consequently, in medical institutions such as hospitals, when the liquidHe is injected at the initial start-up of the MRI apparatus 50, it isnot necessary to regularly add the liquid He, which is expensive and noteasy to handle, in the subsequent operation. Due to this significantimprovement in convenience, the introduction of MRI apparatus 50 tosmall and medium-sized hospitals is expanding. In addition, an MRIapparatus including a direct cooling type superconducting coil, whichconducts and cools the superconducting coil with a refrigerator withoutusing the liquid He, has also been commercialized. In the case of suchan MRI apparatus, the liquid He bath 6 can be omitted.

In recent years, MRI apparatuses using high-temperature superconductingwires such as Y-type, Bi-type, and MgB₂ have been developed. In alsothese apparatuses similarly to the MRI apparatus using a low-temperaturesuperconducting material, the superconducting coil needs to be equal toor lower than the critical temperature of the superconducting transitionand needs to be cooled below 10K to 30K (about −257° C.) at which thecurrent required to generate the magnetic field can flow.

Thus, in the MRI apparatus using a high-temperature superconductingmaterial, it is necessary to cool the superconducting coil by applyingconduction cooling on the superconducting coil with the use of arefrigerator or by submerging it in liquid He, liquid H₂, and/or liquidNe, liquefaction temperature of each of which is 4K to 30K (about −269°C.) or lower under 1 atm. Also in the latter method, it is desirable tocool it by using a refrigerator in order to prevent evaporation ofliquids He, liquid H₂, and liquid Ne. In order to improve theperformance of the refrigerator at 10K to 30K, it is preferred to mounta cold storage material having a large specific heat in the sametemperature region on the refrigerator.

The device incorporating a superconducting coil (i.e., an apparatus witha built-in superconducting-coil) according to the third embodiment isequipped with the refrigerator of the second embodiment, which isprovided with the cold storage material of the first embodiment. Themagnetization of this cold storage material is 10 emu/g or less, morepreferably 5 emu/g or less, and further preferably 2 emu/g or less at anexternal magnetic field of 1000 Oe and a temperature of 5K or lower.Since the magnetization of the cold storage material is small asdescribed above, the influence of magnetic noise derived from the coldstorage material can be reduced and a high-quality image can beobtained. The device incorporating a superconducting coil according tothe third embodiment is not limited to the above-described MRI apparatus50. The device incorporating a superconducting coil according to thethird embodiment includes a superconducting magnet for a magneticallylevitated train, a superconducting magnet device, a cryopump device, aJosephson voltage standard device, and a magnetic-field application typemonocrystal pulling device.

In particular, the cryopump device achieves a high degree of vacuum bybeing cooled to about 10K. Thus, the performance of the cryopump devicecan be improved by mounting a cold storage material having a largespecific heat in the vicinity of 10K on the refrigerator.

EXAMPLES Example 1, Comparative Example 1

Next, Example 1 will be described in more detail. The elemental metals,which are components of the intermetallic compound DyCu₂Ge₂, are used asthe raw materials and are mixed in its stoichiometric ratio so as to bemelted. Further, the distance between the nozzle and the roll is set to0.5 mm under the roll quenching method, and a flaky sample was preparedby rapid-solidification at a cooling rate of 10⁵° C./sec to 10⁶° C./sec.As Comparative Example 1, a bulk sample was prepared by slowly coolingand solidifying the raw materials at a cooling rate of 10²° C./sec usingthe arc melting method under the same compounding and melting conditionsas Example 1.

Example 2

A flaky sample was prepared under the same conditions as in Example 1except that the distance between the nozzle and the roll was set to 0.6mm.

Example 3

A flaky sample was prepared under the same conditions as in Example 1except that the distance between the nozzle and the roll was set to 0.7mm.

FIG. 5 is a graph showing measurement results by a powder X-raydiffraction method for Example 1 in the upper part and for ComparativeExample 1 in the lower part. The measurement of the powder X-raydiffraction was performed by using SmartLab manufactured by Rigaku Co.,Ltd. From the X-ray diffraction pattern of this graph, it can be seenthat most of the crystal structure of the intermetallic compound ofExample 1 obtained by the rapid-solidification treatment is DyCu₂Ge₂. Itcan be seen that the intermetallic compound of Comparative Example 1obtained by the slow-cooling solidification treatment also contains aplurality of subphases.

FIG. 6 is a graph showing the specific heat characteristics of Example 1and Comparative Example in the extremely low temperature region. Thespecific heat characteristics were measured with the use of a physicalproperty measurement system (PPMS) manufactured by Quantum Design Japan,Inc. As shown in FIG. 6 , it can be seen that Example 1 subjected to therapid-solidification treatment is larger in local maximum value of thespecific heat in the low temperature region than Comparative Example 1subjected to the slow-cooling solidification treatment. As a result, thecooling capacity of the refrigerator is improved by adopting theintermetallic compound of Example 1 as the cold storage material to bepacked in the cold storage device of the refrigerator.

Comparative Example 2

Under the same compounding and melting conditions as in ComparativeExample 1, a bulk sample was prepared by heat treatment at 800° C. belowthe solidifying point for one week. The sample preparation conditions ofComparative Example 2 reproduce the above-described disclosureconditions of Non-Patent Document 1.

Comparative Example 3

A bulk sample was prepared under the same conditions as in ComparativeExample 2 except that the heat treatment was performed at 900° C. belowthe solidifying point for four days.

Comparative Example 4

A bulk sample was prepared under the same conditions as in ComparativeExample 2 except that the heat treatment was performed at 800° C. belowthe solidifying point for four days.

Comparative Example 5

A bulk sample was prepared under the same conditions as in ComparativeExample 2 except that the heat treatment was performed at 700° C. belowthe solidifying point for four days.

Comparative Example 6

Under the same compounding conditions as in Example 1, a bulk sample wasprepared by performing slow-cooling solidification at a cooling rate of10²° C./sec under the high-frequency dissolution method.

Example 4

A flaky sample was prepared under the same conditions as in Example 1except that the composition was DyCu₂Si₂.

Comparative Example 7

A bulk sample was prepared under the same conditions as in ComparativeExample 1 except that the composition was DyCu₂Si₂.

Comparative Example 8

A bulk sample was prepared under the same conditions as in ComparativeExample 7 except that the heat treatment was performed at 900° C. belowthe solidifying point for four days.

Example 5

A flaky sample was prepared under the same conditions as in Example 1except that the composition was GdCu₂Si₂.

Comparative Example 9

A bulk sample was prepared under the same conditions as in ComparativeExample 1 except that the composition was GdCu₂Si₂.

Comparative Example 10

A bulk sample was prepared under the same conditions as in ComparativeExample 9 except that the heat treatment was performed at 900° C. belowthe solidifying point for four days.

Example 6

A flaky sample was prepared under the same conditions as in Example 1except that the composition was PrCu₂Si₂.

Comparative Example 11

A bulk sample was prepared under the same conditions as in ComparativeExample 1 except that the composition was PrCu₂Si₂.

Comparative Example 12

A bulk sample was prepared under the same conditions in ComparativeExample 11 except that the heat treatment was performed at 900° C. belowthe solidifying point for four days.

Example 7

A flaky sample was prepared under the same conditions as in Example 1except that the composition was NdCu₂Si₂.

Comparative Example 13

A bulk sample was prepared under the same conditions as in ComparativeExample 1 except that the composition was NdCu₂Si₂.

Comparative Example 14

A bulk sample was prepared under the same conditions as in ComparativeExample 13 except that the heat treatment was performed at 900° C. belowthe solidifying point for four days.

FIG. 7 is a graph showing measurement results by the powder X-raydiffraction method for Example 1 in the upper part and for ComparativeExample 2 in the lower part. Example 1 shown in the upper part of FIG. 7and Example 1 shown in the upper part of FIG. 5 are the same data exceptthe scale display on the horizontal axis. As shown in FIG. 7 , it can beseen in Comparative Example 2 that the X-ray diffraction pattern of theunintended crystal structure being present in Comparative Example 1disappears and most of the crystal structure becomes DyCu₂Ge₂ similarlyto Example 1 by maintaining a high temperature and heat-treating thesolid phase.

Comparing the X-ray diffraction patterns in FIG. 7 between Example 1 andComparative Example 2, it can be seen that the peak spread is larger inExample 1. The peak identified as ThCr₂Si₂-type structure was used forcalculating the crystallite size from the half width β. Even if thecrystal structure of the intermetallic compound is the same, Example 1subjected to rapid-solidification treatment is smaller in crystallitesize than Comparative Example 2 subjected to heat-treatment at a hightemperature in a solid phase, and has excellent mechanical properties.

The samples were put into a container (D=15 mm, h=14 mm) of a vibrationtester, and a simple vibration with a maximum acceleration of 300 m/s²was applied 1×10⁶ times by the vibration tester. After this test, themechanical strength of each sample was evaluated by appropriatelyclassifying the samples in terms of shape and sieving the samples and bydetermining the weight ratio of each pulverized sample.

FIG. 8 is a table of results showing crystallite size, contentpercentage of ThCr₂Si₂-type structure, proportion of pulverized sample,peak temperature of specific heat, and peak value of specific heat inthe samples of Example 1 to Example 7 and Comparative Example 1 toComparative Example 14. When the crystallite size is larger than 70 nm,the proportion of pulverization is significantly increased and themechanical strength is reduced. When the content percentage of theThCr₂Si₂-type structure is less than 801; by volume, the peak value ofspecific heat is significantly reduced.

FIG. 9 is a graph showing the magnetization characteristics in theextremely low temperature region for Example 1 The magnetizationcharacteristics were measured with the use of a magnetic propertymeasurement system (MPMS) manufactured by Quantum Design Japan, Inc.When the external magnetic field is 1000 Oe, the magnetization in thetemperature region 2K to 5K is 0.97 emu/g or lower. The magnetization ofDOS, which has almost the same high specific heat characteristics as inExamples 1 to 3 in the temperature region near 5K, is 1.5 emu/g, themagnetization of HoCu₂ used on the low-temperature side of the secondcold storage device excluding GCS is 3.5 emu/g, and the magnetization ofEr₃Ni is 7 emu/g. Accordingly, the cold storage materials of Examples 1to 3 have small magnetization characteristics, and thus contribute toimprovement of image quality and reduction of magnetic noise of thedevice incorporating a superconducting coil when being mounted on an MRIapparatus.

In the case of the cold storage material of Example 1 having a granularparticle size below 0.01 mm (10 μm), the gap between the particles ofthe cold storage material (i.e., space through which the working gasflows) becomes narrower, and the pressure loss of the gas increases,which deteriorates the refrigeration performance. When the granularparticle size of the cold storage material is larger than 1 mm, thefilling rate of the cold storage material in the cold storage devicedecreases, and thus the refrigeration performance deteriorates.

According to the cold storage material of at least one embodiment asdescribed above, a cold storage material, which has a large specificheat and a small magnetization in an extremely low temperature regionand has satisfactory manufacturability, can be provided. Additionally, arefrigerator having high efficiency and excellent cooling performancecan be provided by filling the refrigerator with this cold storagematerial. Further, a device incorporating a superconducting coil capableof reducing the influence of magnetic noise derived from the coldstorage material can be provided.

While certain embodiments have been described, these embodiments havebeen presented by way of example only, and are not intended to limit thescope of the inventions. These embodiments may be embodied in a varietyof other forms, and various omissions, substitutions, and changes may bemade without departing from the spirit of the inventions. Theseembodiments and their modifications are included in the accompanyingclaims and their equivalents as well as included in the scope and gistof the inventions.

REFERENCE SIGNS LIST

-   11 ThCr₂Si₂-type structure-   12 Th site-   13 Cr site-   14 Si site-   15 projected image-   16 circumscribed circle-   30 refrigerator-   31 first cylinder-   32 second cylinder-   33 passage of working gas-   34 first cold storage device-   35 second cold storage device-   36, 37 seal ring-   38 first cold storage material-   39 passage of working gas-   40 (40 a, 40 b) second cold storage materials-   41 first expansion chamber-   42 second expansion chamber-   43 first cooling stage-   44 second cooling stage-   45 compressor-   46 high-pressure line-   47 low-pressure line-   48 mesh-   50 MRI apparatus-   51 bore space-   52 subject-   53 first electromagnet-   54 second electromagnet-   55 RF coil-   56 He bath-   57 adiabatic vacuum layer-   58, 59 shield

1-5. (canceled)
 6. A method of manufacturing a cold storage materialcomprising: a process of blending and melting component elements of anintermetallic compound capable of forming a ThCr₂Si₂-type structure at astoichiometric ratio of the intermetallic compound to obtain a moltenliquid; and a process of rapidly cooling and solidifying the moltenliquid into granules by injecting the molten liquid into a dynamiccooling medium.